1. Field of the Invention
The invention relates to image intensifiers, particularly with respect to second generation and above gated image intensifiers having a microchannel plate with a high voltage microchannel plate input (MCPin). The invention is particularly concerned with gating the photocathode (PC) of the image intensifier over a wide dynamic range.
2. Description of the Prior Art
The prior art has contemplated the use of a gated image intensifier as an electronic shutter. Voltages applied to the photocathode thereof are varied to switch the photocathode on and off. Utilizing the gated photocathode of an image intensifier advantageously provides significantly faster shutter speeds compared to traditional mechanical shutters. At best, the minimum shutter open time for a mechanical shutter is on the order of 100 microseconds. The shutter speed for a gated image intensifier is limited by the propagation time of the charge across the area of the photocathode. For a typical second generation, microchannel plate, 18 millimeter photocathode, image intensifier, the limiting shutter speed is less than 15 nanoseconds. Shutter speeds of this order of magnitude are desirably utilized to stop action in high speed event imaging. For example, travelling projectiles, such as bullets, can be stopped in mid flight. Events such as explosions and plasma flows can be stopped and observed on a stationary frame.
The gated image intensifier may utilize the gate as a light level control by providing a variable gate time. An all light level television camera system utilizes the video output from an intensified camera to pulse width modulate (PWM) the gate of the image intensifier to provide eight orders to magnitude of light level control dynamic range. Such a system is described in the Proceedings of the SPIE, Volume 832, "Dual Mode Auto-Light Control for Low Light Television" by L. H. Gilligan and D. W. Gerdt, Page 307. Additionally, a paper in the Proceedings of the SPIE, Volume 693, "Auto-Light Control by Pulse Width Modulation" by L. H. Gilligan and D. W. Gerdt, Page 53 also discloses such a system. Further details of such systems may also be found in U.S. Pat. No. 4,202,014, issued May 6, 1980, entitled "Pulse Modulated Automatic Light Control" by Gilligan and Hermansdorfer and in U.S. patent application Ser. No. 259,829, filed Oct. 19, 1988, entitled "Automatic Brightness Control for Image Intensifiers" by Gilligan and Gerdt. Said U.S. Pat. No. 4,202,014 and U.S. application No. 259,829 are assigned to the assignee of the present invention.
Many other applications exist for a gated image intensifier. For example, a range gated intensified television camera may be utilized for penetration of atmospheric obscurants such as fog and smoke. Such a system is disclosed in U.S. patent application No. 288,549, filed Dec. 22, 1988, entitled "Atmospheric Obscurant Penetration Target Observation System with Range Gating" by Gilligan and Gerdt. Said Ser. No. 288,549 is assigned to the assignee of the present invention.
Although gating circuits exit in the prior art for gating the photocathode of such gateable image intensifiers, the circuits suffer from numerous disadvantages. Such circuits tend to exhibit low reliability and narrow dynamic range as well as tending to be excessively expensive. These disadvantages accrue because the photocathode (PC) voltage that gates the intensifier on is approximately 80 volts more negative than the microchannel plate input voltage which is typically at a potential of -800 VDC. The gating pulse applied to the PC to gate the photocathode on and off is therefore difficult to generate. The prior art gating circuits utilize very high voltage switching components to gate the photocathode, which components tend to be excessively expensive. Another problem of prior art circuits is the provision of sufficient peak current to charge the PC intrinsic capacitance quickly enough to provide short rise and fall times. Additionally, in order to accommodate the wide dynamic range desired for such all light level television cameras and to provide adequate protection to the image intensifier from excessive light input, gating pulses ranging from 100 nanoseconds through the microsecond and millisecond range to DC should be utilized. The prior art gating circuits do not have the capability of generating sufficiently short pulses to compensate for very high light levels while providing for long on times such as tens of milliseconds or continuous operation for low light levels. Different types of drive circuitry are required for fast nanosecond pulses than for slow rise and fall time millisecond range pulses and for DC operation. Thus, the prior art gating circuits tended to be limited in dynamic range.
Another problem encountered in present day image intensifiers is that if elements of excessive brightness are viewed in a dark scene, photocathode damage may result by the scene tending to "burn in" to the photocathode. Thus, image intensifiers tend to burn in images that are too bright. For example, if the intra-scene dynamic range is too large; e.g., a runway light on a dark runway at night, the bright spot from the light forms a negative image on the photocathode that does not dissipate when power is removed. The burned in images obscure subsequent images and accumulate until the image intensifier picture is no longer useable. This type of damage tends to be reversible if it is not too extreme. It is believed that the damage is caused by impurity ions, generally potassium, transferred from the microchannel plate to the photocathode. resulting in regions of low sensitivity corresponding to the bright regions of the scene. If the damage caused by the damage event is reversible, ion diffusion in the photocathode will obscure or negate the damage over a long period of time. It has been observed that the immediate application of the photocathode of a sufficiently large reverse off potential repairs the ion damage in a relatively short period of time. This problem is discussed in said SPIE paper, volume 832, as well as in said Ser. No. 259,829. The prior photocathode gating circuits do not include means for repairing such damage.